A Toronto startup is hawking a mobile phone add-on that lets users communicate without any cellular signal or wireless network whatsoever. The Sonnet, launching its crowdfunding campaign today, uses long-range radio frequency bandwidth to connect cell phones over a range of several miles.

The Sonnet turns your phone into a walkie-talkie, essentially — or a two-way radio, as the grown-ups call it. It's all built on existing technology (old-fashioned technology, really) but the Sonnet adds some interesting additional functionality.

The hardware itself is a rugged plastic hexagon about the size of small hamburger. Clip the gizmo to your clothes or backpack and it communicates wirelessly with your phone. On the software end, the Sonnet provides a built-in app so you can send text messages, voice recordings, images and GPS coordinates to any other Sonnet-equipped phone within range — encrypted end-to-end with the industry-standard AES protocol.

Just as you would send and receive audio in a two-radio connection, the Sonnet system lets you send and receive data packages — those pictures, texts or recordings. The Sonnet hexagon works as a permanent transceiver; a dedicated walkie-talkie connection that you control with your smart phone.

“Walkie-talkies only transmit low-fidelity audio data, which can often lead to intercommunication,” Boken Lin, founder and CEO of Sonnet Labs, wrote in an email. “This is why cops have a code for everything, to communicate more effectively over walkie-talkies. In addition, there are some information such as GPS coordinates that are much more effective when sent over the air as text.”

The idea is to give mobile phone users another option in situations where they can't get access to the cellular infrastructure. Hiking groups will likely find this option most useful, but you could also switch over when you can't get a signal at a crowded convention or festival. Sonnet is also a backup communication system for worst-case scenarios — a natural disaster, say, or the zombie apocalypse.

Under normal outdoor conditions, point-to-point range is about three miles, according to the company. If you can manage line-of sight, that range can be doubled. Indoors or in the city, range drops to about one mile. In large groups, the Sonnet's mesh network technology kicks in, turning each Sonnet device into a relay and signal booster, further expanding the range.

The rechargeable lithium-polymer battery lasts up to 24 hours, and in a pinch you can use a USB cable to power your phone from the Sonnet device.

The Sonnet's GPS function can be used with a database of offline maps, stored within the unit itself. If you're out hiking, the Sonnet app can track the location of others in your party and you can leave digital “breadcrumbs” to mark your routes. For genuine emergencies, the Sonnet has a wideband SOS feature that puts out a distress call on all frequencies.

Lin said inspiration for the Sonnet device came after hiking with friends in Ontario's Algonquin Park.

“The park is huge and has many entrances, and the biggest problem was figuring out a place to meet up before entering the park together,” Lin said. “Sometimes we would use landmarks but it got confusing. So Sonnet really started out as just a way to share our GPS coordinates. Once we built out the technology, though, we realized that it can be used for a lot more than just GPS coordinates.”